Demand for wireless communication services, such as mobile telephones in cellular and Personal Communications Service (PCS) systems, is continually increasing. An important issue in wireless communication systems involves the estimation of mobile station location. For example, the Federal Communications Commission (FCC) has requested that all cellular and PCS systems eventually include emergency 911 caller location capabilities similar to those provided in wired systems. As reported in Radio Communications Report, Vol. 15, No. 51, Dec. 16, 1996, the FCC has required that Phase I of a wireless emergency 911 (E-911 ) system providing a 911 agent with caller number and cell site location must be completed by Apr. 1, 1998, while Phase II of the E-911 system providing caller latitude and longitude within a radius of no more than 125 meters in at least 67% of all cases must be completed by Oct. 1, 2001. A number of other services requiring mobile location estimation are also being considered, including routing guidance services, fleet management and local commercial services. A wireless system which is able to determine the position of a given mobile station in an efficient manner could thus provide an enhanced level of service to the user, while meeting the above-noted FCC requirements and also generating additional revenue for the service provider. It would also be very advantageous if mobile location estimation could be incorporated into a given system without the need for any significant change to the standards on which the system is based.
In order to estimate mobile location with an acceptable degree of accuracy, either the mobile station needs to be able to detect signals from at least three surrounding base stations, or at least three surrounding base stations need to be able to detect a signal from the mobile station. The resulting signal propagation delay information can then be processed in a conventional manner using triangular relationships to derive an estimate of mobile location. Two important aspects of mobile location estimation thus involve the manner in which the signals are detected in the mobile station or base stations, and the accuracy of the propagation delay measurements required between the mobile station and base stations. An exemplary prior art mobile estimation technique is described in M. Wylie et al., "The Non-Line of Sight Problem in Mobile Location Estimation," ICUPC '95, Boston, Mass., 1995, which is incorporated by reference herein.
In the case of code division multiple access (CDMA) systems such as those based on the IS-95 standard, the implementation of a mobile location estimation capability presents a number of problems. Although CDMA systems spread signals over a wider frequency spectrum than narrowband systems such as TDMA, GSM and analog FM, and are therefore better able to resolve path delay ambiguity, the detection of signals from surrounding base stations by the mobile or detection of mobile signals by the surrounding base stations presents a greater challenge in CDMA systems than in narrowband systems. A basic principle of CDMA systems involves the use of power control to solve near-field problems and to control interference such that a capacity advantage can be achieved. Therefore, when the mobile station is not in a handoff zone, that is, when the mobile station is close to a base station, the signal strength from the surrounding base stations is very weak. Similarly, the power transmitted from the mobile is purposely made very small in order to prevent interference. This means that in order to achieve the capability of detecting signals from surrounding base stations in the mobile station or detecting a signal from the mobile station in surrounding base stations, an excessively large signal-to-noise gain may be required at the corresponding receivers. Application of conventional mobile location techniques to CDMA systems may therefore require alteration of basic system parameters, thereby increasing the cost and complexity of the system and possibly degrading system performance in terms of interference. An exemplary CDMA mobile estimation technique is described in J. Caffery et al., "Radio Location in Urban CDMA Microcells," Proceedings of PIMRC '95, pp. 858-862, IEEE, 1995, which is incorporated by reference herein. There are a number of problems with this prior art technique and other similar techniques. For example, such techniques typically utilize either coarse timing acquisition or a finer-acquisition delay lock loop to obtain path delay information. As noted above, it is difficult to detect a mobile station signal at the surrounding base stations unless an excessively large signal-to-noise gain is obtained, and the coarse timing and delay lock loop techniques have failed to solve this problem. In addition, the use of a delay lock loop usually requires the surrounding base stations to detect the mobile station signal either continuously or for a very long detection period, thereby wasting system resources and significantly increasing system complexity. Moreover, it is generally not feasible to utilize a delay lock loop in conjunction with an increased mobile station power level to assist acquisition because the resulting interference would substantially reduce system capacity.
As is apparent from the above, a need exists for an improved technique for estimating mobile station location in a wireless communication system, and which can be implemented in a CDMA system without requiring any significant alteration to system operating and performance standards.